I have several buttons on a gui, each with its own specialized callback. The GUI responds quickly to most of them. However, if one of them (next button) is the first thing pressed, the GUI takes a long time (several seconds) to respond. I put a disp('In next button callback'); at the top of the callback just to see when it enters and this output doesn't even show up for a while. Strangely, this is only true for this button and not for the others, even though they look to be set the same way.